In ac circuit, there are three concepts of electric power: active power, reactive power and apparent power. KVA stands for apparent power, which includes reactive power and active power. KW stands for active power, and kVar stands for reactive power. The relation between kW and kVar and conversion there is a concept - power factor cos ф, active power kW=UIcos ф, reactive power kVar=UIsin ф, and apparent power kVA=UI(U is voltage, I is current).
About transformer capacity selection, there are the following rules for reference!
Given the transformer capacity, find its voltage grade side rated current
Description: Suitable for any voltage level.
Formula: capacity divided by voltage, multiplied by six divided by ten.
Case :
Apparent current I= apparent power S/1.732 * 10KV=1000KVA/1.732 * 10KV=57.736A
Estimated I=1000KVA/10KV * 6/10=60A
Known transformer capacity, quick calculation of primary and secondary protection fuses current value
Transformer high voltage fuse, capacity voltage comparison.
With variable low voltage fuse, capacity multiplied by 9 divided by 5
3. Measure the secondary current of the power transformer and calculate its load capacity
Known distribution variable secondary voltage, measured current kilowatts.
Voltage class 400 volts, one ampere 0.6 kilowatts.
Voltage clas 3,000 volts, 1 amp 4.5 kilowatts.
Voltage class 6,000 volts, one amp integer nine kilowatts.
Voltage class 10 kv, 1 amp 15 kW.
Voltage level 35,000, 1 amp 55 kilowatts.
4. Given the transformer capacity, find the rated current of its voltage grade side
Common voltage coefficient, capacitive multiplication coefficient of the current,
Rated at 400 volts, coefficient 1.445,
Rated voltage 6000 volts, coefficient 0. 096,
Rated at 10,000 volts, coefficient is just 0.6.
Note: the rated current of the voltage grade side can be obtained by multiplying the transformer capacity directly by the corresponding coefficient.
